It doesn't matter if you hold the original WP document and don't return it. If your employer ends your job, the WP is no longer valid. If your employer wants to comply exactly with the law they can submit the document below to the Home Office notifying them that your employment is over. Most companies don't do this promptly, many don't do it at all.
http://www.workingintheuk.gov.uk/workin ... p/NPEE.doc
If your employer sends this form in (not very nice are they?), and the Home Office is on top of things (usually they are not so efficient), the HO will send you a letter notifying you to leave the country in 28 days. If you receive such a letter and do not have a new employer who can file a new work permit in that time, should leave the UK before the time is up to avoid violating the immigration laws. If you are a non visa national you can always reenter as a visitor to continue your job search. Do not say you are still working, even if your current passport WP stamp shows plenty of validity. This would be lying, and if you are caught lying (they will show the cancellation on their database) you will be refused entry have long term problems ever entering the UK.
If you find one quickly (within a few months), you can get a new workpermit and probably not lose the time toward ILR. You may need to use an immigration solicitor to ensure that this is done right.